1. Field
The present disclosure relates generally to circuits for driving light-emitting diodes (LEDs) and, more specifically, to LED driver circuits having phase-angle dimming circuitry.
2. Related Art
LED lighting has become popular in the industry due to the many advantages that this technology provides. For example, LED lamps typically have a longer lifespan, pose fewer hazards, and provide increased visual appeal when compared to other lighting technologies, such as compact fluorescent lamp (CFL) or incandescent lighting technologies. The advantages provided by LED lighting have resulted in LEDs being incorporated into a variety of lighting technologies, televisions, monitors, and other applications.
It is often desirable to implement LED lamps with a dimming functionality to provide variable light output. One known technique that has been used for analog LED dimming is phase-angle dimming, which may be implemented using either leading-edge or trailing-edge phase-control. A Triac circuit is often used to perform this type of phase-angle dimming and operates by delaying the beginning of each half-cycle of alternating current (ac) power or trimming the end of each half-cycle of ac power. By delaying the beginning of each half-cycle or trimming the end of each half-cycle, the amount of power delivered to the load (e.g., the lamp) is reduced, thereby producing a dimming effect in the light output by the lamp. In most applications, the delay in the beginning of each half-cycle or trimming of the end of each half-cycle is not noticeable because the resulting variations in the phase-controlled line voltage and power delivered to the lamp occur more quickly than can be perceived by the human eye. For example, Triac dimming circuits work especially well when used to dim incandescent light bulbs since the variations in phase-angle with altered ac line voltages are immaterial to these types of bulbs. However, flicker may be noticed when Triac circuits are used for dimming LED lamps.
Flickering in LED lamps can occur because these devices are typically driven by LED drivers having regulated power supplies that provide regulated current and voltage to the LED lamps from ac power lines. Unless the regulated power supplies that drive the LED lamps are designed to recognize and respond to the voltage signals from Triac dimming circuits in a desirable way, the Triac dimming circuits are likely to produce non-ideal results, such as limited dimming range, flickering, blinking, and/or color shifting in the LED lamps.
The difficulty in using Triac dimming circuits with LED lamps is in part due to a characteristic of the Triac itself. Specifically, a Triac is a semiconductor component that behaves as a controlled ac switch. Thus, the Triac behaves as an open switch to an ac voltage until it receives a trigger signal at a control terminal, causing the switch to close. The switch remains closed as long as the current through the switch is above a value referred to as the “holding current.” Most incandescent lamps draw more than the minimum holding current from the ac power source to enable reliable and consistent operation of a Triac. However, the comparably low currents drawn by LEDs from efficient power supplies may not meet the minimum holding currents required to keep the Triac switches conducting for reliable operation. As a result, the Triac may trigger inconsistently. In addition, due to the inrush current charging the input capacitance and because of the relatively large impedance that the LEDs present to the input line, a significant ringing may occur whenever the Triac turns on. This ringing may cause even more undesirable behavior as the Triac current may fall to zero and turn off the LED load, resulting in a flickering effect.
To address these issues, conventional LED driver designs typically rely on current drawn by a dummy load or “bleeder circuit” of the power converter to supplement the current drawn by the LEDs in order to draw a sufficient amount of current to keep the Triac conducting reliably after it is triggered. These bleeder circuits may typically include passive components and/or active components controlled by a controller or by the converter parameters in response to the load level. While useful to sink additional current, a bleeder circuit that is external to the integrated circuit requires the use of extra components with associated penalties in cost and efficiency.